10. Rocks and Fossils
The Geological Record: Reading Rocks

In many ways, rocks and fossils are the most perfect form of scientific evidence. When rocks solidify, they lock in a record of their environmental conditions, preserving geological information for millions or even billions of years. Similarly, everything we know about life that has existed on our planet for hundreds of millions of years comes from the fossils left behind. These rocks and fossils tell the incredible story of the Earth’s past! Continents colliding, strange plants and animals inhabiting the seas, gigantic creatures roaming the Earth, and enormous pterosaurs soaring overhead. Best of all, almost any physically fit person can directly confirm many of these mind-boggling facts by going on field trips in search of rocks and fossils.
Of the three basic types of rocks—igneous, sedimentary, and metamorphic—we are most interested in sedimentary rocks. As a quick geology refresher: igneous rocks form from magma rising from the Earth’s interior. When this hot, liquid material reaches the surface, it solidifies as igneous rock. Volcanic magma can either erupt into the atmosphere or flow as lava from a volcanic opening. Sometimes, the magma doesn’t reach the surface and instead cools slowly underground, allowing minerals to crystallize into plutonic rocks such as granite. Regardless of how igneous rocks form, once exposed to the surface they are gradually worn down by erosion. These eroded fragments are eventually washed away and settle to form sedimentary rock layers. Sedimentary rocks can also form through other processes, such as chemical precipitation, salt deposition, and the accumulation of marine fossils. The third type of rock, metamorphic rock, forms when igneous or sedimentary rocks are buried deeply within the Earth. While buried deeply beneath the surface these rocks are exposed to intense heat and pressure that cause changes in their crystalline structure thus transforming them into metamorphic rocks. Of these three types of rocks — igneous, sedimentary, and metamorphic — the layering of the sedimentary rocks allows us to give a time-line to the evidence preserved in the sedimentary rocks thus giving us Earth’s geological history and this is why these rocks are the most important for our study.

Because rocks are so incredibly common, there is a strong tendency to take them for granted. Yet, to a trained geologist, common rocks hold a wealth of information. The types of rocks found at a site, how well they are sorted, and the wear marks they exhibit can provide valuable insights into a region's sedimentary deposits. Similarly, evidence such as glacial scraping, windblown deposits, mud cracks, and ripple marks reveals details about past climates. However, all of this information is of limited value if we don't know the age of the rocks. Fortunately, we can determine a rock's age by applying our understanding of how sedimentary layers form.
Two of the most fundamental principles in geology are the law of original horizontality and the law of superposition. Together, these principles explain that sedimentary rocks are initially deposited in horizontal layers, much like a stack of pancakes. And just like a pancake stack, the youngest sedimentary layer is always the last one added. This means that as we drill deeper into the Earth, we are essentially traveling back in time, passing through successively older rock layers. In nearly all cases, the oldest rock layers will be at the bottom of the stack.

As water flows downhill, it carries eroded rock material from higher elevations to lower elevations, where the sediment is deposited and forms new rock layers. These sedimentary layers accumulate in low-lying areas such as valleys, lakes, floodplains, and river deltas. However, while new sedimentary rock layers are being formed, older layers are simultaneously being erased or destroyed.
Interestingly, most mountains are composed of sedimentary rock because they were once part of lower-elevation landscapes. These mountains often form through uplift caused by the collision of continents. Additionally, sedimentary rock can be disrupted or lost due to the oscillation of continents during the advance and retreat of ice age glaciers. Each time the glaciers retreat, the continents rise again, accelerating erosion.
Even under ideal conditions for continuous sediment deposition, the deepest layers would eventually become buried so deeply that heat and pressure would transform them into metamorphic rock. This process erases much, if not all, of their geological information. Consequently, a geologist cannot simply dig or drill downward at a single location to reconstruct Earth's history all the way back to its beginnings.

If we think of sedimentary rock layers as the pages of a book, there is no single place on Earth where we can continuously read this geological "book" from the present day back to the beginning — or even come close. Nevertheless, by piecing together information from multiple locations, we can still delve remarkably far into Earth's past. Fossils play a pivotal role in this process, allowing geologists to chronologically match sedimentary rock layers from one site with those at another.
Fossils — the remnants of ancient life and the impressions left in rocks — reveal even more about Earth’s geological history than the rocks themselves. From the tiniest microscopic species to the largest organisms, fossils provide insights into everything from local paleoclimates to the discovery of valuable resources like oil. Without fossils, we would have no record of dinosaurs or the countless other fascinating species that once thrived on our planet.
One of the most significant attributes of fossils is their ability to mark geological time. Over the past half-billion years, numerous species achieved worldwide distribution, thrived for a few million years, and then went extinct. During their existence, the remains of these species were embedded in sedimentary rock as it formed. This process allows geologists to link sedimentary layers from different locations around the world chronologically, provided they contain the same key index fossils.
Geologists have also discovered a consistent order to the placement of fossils in rock layers — certain species’ fossils are always found above or below others. Since younger rock is deposited on top of older rock, this vertical arrangement reflects the chronological sequence of species. When geologists encounter this pattern or even a single key fossil, they can determine the relative age of the rock layer with remarkable confidence.

By comparing present-day geological processes with evidence preserved in ancient rock layers, geologists of the 18th and 19th centuries concluded that Earth is incredibly old. However, the exact age of the Earth remained a mystery. Fossil evidence alone could establish the chronological order of events but not their precise timing. It wasn’t until the 20th century, with the development of radioactive dating methods, that geologists could determine the absolute ages of rock layers with accuracy.
In addition to fossils, radioactive material is sometimes found embedded within sedimentary rock layers. Like fossils, this material can be used to determine the age of the rocks. Once formed, the radioactive material of an element decays exponentially into a new substance, referred to as its daughter element. The rate of this decay is determined by the decay constant (λ), which dictates whether the process takes less than a second or spans billions of years.
To better understand the pace of this transformation, the decay constant is often expressed as the element's half-life. The half-life is the amount of time required for half of the radioactive material to decay into its daughter element. By knowing the half-life of a radioactive element and measuring the percentage of radioactive nuclei remaining in a sample, technicians can determine the sample's age.
When radioactive dating methods are applied to sedimentary rock layers, they also reveal the age of the fossils contained within those layers. Repeated analyses of numerous samples have established the absolute ages of many key fossils. With this information, a geologist in the field can identify one or more of these key fossils in the rock layers and instantly determine both the chronological and absolute age of the rock.

Around 544 million years ago, single-celled life evolved into a diverse array of organisms capable of leaving behind fossilized evidence of their existence. These fossils define the Phanerozoic Eon, whose name means "visible life." As noted earlier, geologists can easily date sedimentary rock layers by identifying the fossils embedded within them. Because sedimentary layers from the Phanerozoic Eon typically contain fossils, while those from earlier eons do not, our understanding of Earth's history during the Phanerozoic is far more detailed than our knowledge of the preceding four billion years.
The Phanerozoic Eon is divided into three eras and eleven periods, with the names of the periods reflecting the distinctive sets of fossils found within each. Each period represents a time when certain fossil species thrived and were unique to that era, providing a clear chronological framework.
This chapter delves into the interplay between environmental conditions, significant events, and the evolution of life on Earth. Specifically, we aim to interpret geological observations through the lens of established scientific principles. For instance, if Darwin's theory of gradual evolution is correct, why does the fossil record often show species appearing suddenly, seemingly out of nowhere? Beyond being known as the "Age of the Dinosaurs," what unique features distinguish the Mesozoic Era? Lastly, how did events during the Phanerozoic Eon influence the thickness of Earth's atmosphere, and how does the atmosphere's thickness, in turn, help explain the fossil evidence?
Evolution and the Fossil Record
In 1859, Charles Darwin introduced his Theory of Evolution with the publication of On the Origin of Species. The idea that new species evolve from preexisting ones was, and remains, a groundbreaking insight into the field of biology. Initially, Darwin developed his ideas on evolution based on evidence he gathered from the Galápagos Islands. Over time, he also recognized that humans had created many new species — such as pigeons, dogs, crops, and livestock — through selective breeding of original species for desirable traits. Additionally, Darwin studied embryonic development and noted that embryos provide evidence of common ancestry among seemingly unrelated species. For example, a human embryo has a prominent tail and closely resembles the embryos of turtles, birds, fish, and other diverse animal groups. Since Darwin's time, advances in DNA analysis have not only confirmed but also refined our understanding of the evolutionary relationships between species.

Darwin's Theory of Evolution is both an outstanding scientific theory and an irrefutable fact. This classification stems from the overwhelming number of independent lines of evidence supporting it. These include the common biochemistry of cells, comparative anatomy, biogeographical patterns, embryological similarities, molecular biology (DNA), genetic commonalities, direct observations, and experimental verifications — just to name a few. The sheer weight of this evidence places the Theory of Evolution on the same level of certainty as the understanding that the Earth is round. Consequently, scientists no longer debate its validity. However, despite this consensus within the scientific community, many people — particularly those with conservative religious beliefs — continue to reject the Theory of Evolution.
Fossils have become a central focus in the public debate over evolution. Creationists argue that the fossil record seems to conflict with Darwin's ideas of gradual evolution, noting that it often shows new species appearing suddenly and remaining unchanged for millions of years before going extinct. In response, scientists have sought — and sometimes found — transitional fossils that illustrate the evolutionary steps linking one species to another. Still, fossils showing little or no change far outnumber transitional ones and so without a more nuanced understanding of how evolution operates the fossil record might seem to support the creationist perspective. Fortunately this improved understanding of evolution came with the publication of Punctuated Equilibrium.
It wasn’t until 1972 that Niles Eldredge and Stephen Jay Gould published their landmark paper on Punctuated Equilibrium, which reconciled the fossil evidence with evolutionary science. To be clear, Punctuated Equilibrium does not overthrow Darwin’s Theory of Evolution; rather, it represents a significant advancement in our understanding of how the evolutionary process operates.

According to Punctuated Equilibrium, new species can evolve rapidly when a small, isolated population of an established species is separated from its larger community. For instance, when a volcanic island emerges above ocean waters, early arrivals — such as birds, insects, and plant seeds — encounter an environment free from significant biological competition. In this new setting, these organisms quickly adapt and evolve to occupy the available ecological niches. If the conditions are favorable, the evolution of new species can occur quite fast. For example, from just one pair of wayward birds, several distinct bird species may evolve in only a few dozen generations.
This was precisely the case for the finches that Charles Darwin observed on the Galapagos Islands. During his expedition, Darwin collected specimens representing fourteen different species of birds. However, upon returning home, his colleague John Gould informed him that all of these birds were finches. It became evident that these species had all evolved from a very small number of early-arriving finches — perhaps a single pregnant female, a pair, or a small flock.
The time required for each reproductive cycle is a key factor in determining how quickly a new species can evolve and grow. This generation time varies widely across species and is largely influenced by the size of the individuals. For instance, the generation time for large animals, such as humans or elephants, can exceed twenty years, while for bacteria, it is typically less than an hour. Despite these differences, even species with long generation times can evolve relatively quickly compared to the geological timescale. For example, a species requiring twenty years per generation could complete fifty generations in just 1,000 years — a span sufficient for the evolutionary changes described by Darwin.

Fifty generations are also enough time for a population to grow exponentially. Consider starting with a single pair of organisms. If each pair of offspring successfully raises six reproducing adults per generation, then by the fiftieth generation, the population could exceed 100,000,000,000,000,000,000,000 individuals.
Once a new species evolves and its population grows, it may remain in its original geographic region or expand into new areas. Like its initial evolution and population growth, this geographic expansion can occur rapidly. A species can evolve and achieve global distribution in just a few thousand years. While this might seem lengthy from a human perspective, it is almost instantaneous on the geological timescale. Sedimentary deposits, which record geological time, accumulate over millions of years. Consequently, a change occurring over a few thousand years appears instantaneous in the fossil record.
This deeper understanding of the evolutionary process explains why the fossil record often shows new species appearing "suddenly." What seems abrupt is, in fact, the result of natural processes occurring over spans of time that are relatively short compared to the vast expanse of geological history.
Now that we have addressed the first geological mystery, the more challenging question is why a species, once evolved, usually remains mostly unchanged for millions of years. Scientists seem overly focused on proving that species evolve, leaving less attention to what locks a species into its form. While all species can evolve, they need environmental opportunities — like unoccupied niches — to do so. Without these, a species often remains in stasis, well-suited to its existing conditions and resistant to change.

To better understand evolution, it is essential to emphasize that evolution has no guidance or intent. For instance, while it may seem advantageous for a species to evolve the ability to fly, no species can consciously choose to evolve in that direction. Evolution only progresses through a series of small, beneficial steps, each of which must improve the species' chances of survival or reproduction. Moreover, even before evolution can occur, a species must overcome inherent resistance to change. For example, an individual with a beneficial trait might fail to pass it on simply because the trait is perceived as socially or physically abnormal within the species' community. Consider a hypothetical case of a person born with three eyes: it does not matter that they have superior vision; they are probably going to have a hard time finding a date for the high school prom.
While biologists often describe how environmental pressures drive evolution, it may be more accurate to frame this as environmental opportunities. When a new ecological niche arises, existing species may adapt to occupy it, or entirely new species may evolve to fill the gap. But what happens when no new niches emerge? In a static environment, evolutionary progress can stall. Without environmental changes, both species' populations and their traits may remain stable for millions of years. In contrast, environments like the Galapagos Islands, with their newly formed volcanic landscapes, provide fertile grounds for the emergence of new species due to the abundance of unclaimed niches.
In addition to geological changes such as volcanic islands creating new uncontested environments, mass extinctions also play a critical role in opening ecological niches. Mass extinctions wipe the environment clean of the old species thus clearing the way for the evolution of new species.
Environment Determines Evolution, Stasis, or Extinction
Despite having the ability to evolve, species will usually not evolve unless there is a significant change in their environment. These environmental changes that affect evolution can occur in three main ways: the creation of nursery environments that foster the evolution of new species, major geological events leading to mass extinctions, and environmental changes driven by the impact of highly successful species. The previous section covered how nursery environments, such as newly formed volcanic islands, encourage the evolution of new species. Now, let us turn to the topic of mass extinctions.

Typically, the eruptions of super volcanoes are the only geological events powerful enough to cause the global devastation associated with mass extinctions. Super volcanoes, also known as traps, are not merely larger versions of regular volcanoes but are cataclysmic events that release approximately a million times more material than the largest eruptions documented in human history. The Siberian Traps are widely regarded as the primary cause of the "Great Dying" at the end of the Permian period, an extinction event that eliminated 85–95% of all terrestrial and marine species.
While most mass extinctions are attributed to super volcanic eruptions, two notable exceptions stand out. Geologists continue to debate whether the K-T extinction, which wiped out the dinosaurs, was caused by a meteor impact, the eruption of the Deccan Traps in India, or a combination of both. The other unusual mass extinction is the ongoing one, which is being driven by human activity.
The effects of mass extinctions are clearly recorded in the geological record. As noted earlier, the fossil record is divided into periods, each representing a time of relative stability. In the Phanerozoic eon, most periods are marked by the occurrence of a mass extinction at either its beginning or its end. These catastrophic events reset the ecological landscape, allowing numerous new species to evolve from the survivors.
While the vast majority of species have little to no impact on their environment, successful species or groups of species can, given enough time, dramatically alter it. For example, Earth is currently undergoing a mass extinction caused by human activity; as we overpopulate the planet we are creating vast amounts of pollutants and depleting the Earth’s resources thus causing the extinction of countless species. Similarly, the evolution of photosynthetic organisms and shell-forming marine animals transformed Earth’s atmosphere from being rich in carbon dioxide to one dominated by nitrogen and oxygen.

A compelling illustration of how species can shape their environment is the popular educational video How Wolves Change Rivers, which documents how the presence or absence of wolves in Yellowstone National Park has had cascading effects on the park’s entire ecosystem. Educating the public about the profound ways species can influence their environment is both necessary and long overdue.
Recognizing that species can dramatically change their surroundings deepens our understanding of the evolution of both life and its environment. When Earth first formed and after each mass extinction, the species that thrived were those able to utilize the resources readily available in the new environment. Initially, these resources likely seemed inexhaustible. However, as certain species grew in population and consumed resources at an exponential rate, their impact became apparent. Over time, many of these dominant species either went extinct due to creating toxic environments or exhausting the resources they depended on.
Our understanding of the relationship between life and its environment has been one-sided: the prevailing view is that the environment dictates the evolution of life, which is subjected to the whims of supervolcanic eruptions and other geological disruptions. While this perspective is valid, it neglects the equally significant role of life in actively transforming its environment. This lack of awareness about how life can change the environment, including the atmosphere, helps explain why Earth’s dramatically different atmosphere, compared to nearby planets, was not initially recognized as being the result of life evolving and existing on Earth.
Earth’s Changing Atmosphere

When the Earth was young, volcanic activity was much more intense than it is today. Initially, the earliest released gases were likely the lighter ones, such as hydrogen and helium, since their buoyancy allowed them to reach the surface first. However, these light gases played little to no role in the development of Earth’s atmosphere because their low molecular weight allowed them to escape Earth’s gravity and drift into space. Later, water vapor and carbon dioxide became the most abundant volcanic gases. While some of the water vapor condensed and formed oceans, much of it was broken apart by photodissociation. The resulting hydrogen escaped Earth's gravity, while the oxygen reacted with minerals on the surface. Meanwhile, carbon dioxide, along with significant amounts of nitrogen and other gases, accumulated in the atmosphere. As volcanic activity gradually decreased, it became more sporadic, leaving the Earth mostly tranquil, punctuated by occasional massive eruptions.
Earth's atmosphere began diverging from the atmospheres of Venus and Mars with the evolution of cyanobacteria and other photosynthesizing organisms. Initially, the production of oxygen, rather than the removal of carbon dioxide, had the most significant impact. Oxygen's presence led to the formation of diatomic and triatomic oxygen in the upper atmosphere, which blocked ultraviolet radiation from reaching the surface. This reduction in UV radiation slowed the destruction of surface water, allowing Earth to retain its abundant water supply — unlike the dry planets Venus and Mars — enabling life to thrive.

While plants remove carbon dioxide in the process of creating carbohydrates through photosynthesis, they were not the primary means by which most of the carbon dioxide was removed from the Earth’s atmosphere. This is because, typically, after plants die, fungi, bacteria, and other detritivores break down and decompose the plant material, returning nutrients such as carbon dioxide and nitrogen to the environment. The exception occurs when the microbial decomposition process is slowed, such as when plant material falls into swampy, anaerobic water. In this case, some of the plant material converts to peat, which later, after burial and significant compression, hardens to become coal. Coal is a fairly abundant resource, so this process must have contributed to reducing the Earth’s carbon dioxide atmosphere. Nevertheless, the amount of carbon locked in coal deposits is still negligible compared to the vast amounts sequestered in carbonate rock.
Most of the carbonated rock on Earth consists of nearly equal amounts limestone and dolomite and together these two types of carbonated rocks comprising about 20 percent of all sedimentary rocks. Geologists remain uncertain about how ancient dolomite layers formed, other than knowing it precipitated from water containing calcium, magnesium, and carbonate ions. It is believed that bacteria, along with possibly elevated temperatures and pressures, played a key role in dolomite formation. In contrast, the numerous marine shell fossils, both large and small, found in limestone make it clear that marine life played a significant role in the formation of most limestone layers. As one might expect, dolomite is more dominant before the Phanerozoic eon, while limestone becomes more dominant during the Phanerozoic eon. Additionally, both are often found in abundance — sometimes in layers, one atop the other — during periods when life was good.
And just what is meant by "when life is good," and what does this have to do with the thickness of the Earth’s atmosphere?
While some limestone formed before the Phanerozoic eon, the vast majority of limestone formation occurred after the Cambrian Explosion. The emergence of marine animals significantly reduced the Earth’s thick carbon dioxide atmosphere while leaving behind a record of their existence in the limestone containing their fossilized shells. Although both dolomite and limestone remove carbon dioxide from the atmosphere, the rate of limestone formation was apparently many times greater than that of dolomite. While dolomite has been forming almost since Earth’s beginning, most limestone formations did not start until nearly four billion years later — and yet today, there are roughly equal amounts of these carbonated rocks.

Thus, it is logical to conclude that the peak of the Earth’s thick atmosphere occurred around the time of the Cambrian Explosion, coinciding with the emergence of marine organisms, the formation of limestone, and the removal of carbon dioxide from the atmosphere. Since the beginning of the Phanerozoic eon, the Earth’s thick, carbon dioxide-dominated atmosphere has been transitioning to today’s relatively thin nitrogen and oxygen atmosphere, with just one major interruption: the "Great Dying" at the end of the Permian period.
Around 252 million years ago, more than 96 percent of marine species were wiped out during the eruption of the Siberian Traps. These marine species had been crucial for removing carbon dioxide from the oceans, which, in turn, reduced carbon dioxide levels in the atmosphere. The Siberian Traps, along with subsequent volcanic activity, released massive amounts of carbon dioxide into the air. At the same time, for tens of millions of years after the eruption, the absence of marine species meant there was no effective mechanism to remove this carbon dioxide.
At the time of the eruption, the Earth had been enjoying a relatively thin nitrogen and oxygen atmosphere, similar to what exists today. However, following this "mother of all mass extinctions," carbon dioxide filled the air, and the atmosphere became extremely thick—resembling conditions from the early Phanerozoic eon. This thick atmosphere set the stage for the Mesozoic era: the age of giants.
Correcting the Eras
The Phanerozoic eon, beginning about 544 million years ago (mya) with the Cambrian Explosion, spans to the present and is divided into the Paleozoic, Mesozoic, and Cenozoic eras, separated by two major mass extinctions: the P-T extinction (~245 mya) and the K-T extinction (~65 mya). These eras are often associated with dominant animal groups: fishes in the Paleozoic, dinosaurs in the Mesozoic, and mammals in the Cenozoic. However, this view oversimplifies history, as fishes thrived throughout the eon, and mammals existed before the Mesozoic. Dinosaurs alone were limited to their era.
While using mass extinctions to define eras is convenient, it overlooks the dramatic changes in global climate throughout the Phanerozoic. Evidence such as ice ages, sea level fluctuations, and global convection patterns aligns with shifts in atmospheric thickness, which directly influenced the permissible size of terrestrial animals. Thus, the thickness of the Earth’s atmosphere may be a better criterion for distinguishing eras than simply the dominant fauna.
A climate-based division of the Phanerozoic would still recognize the Mesozoic as a warm, thick-atmosphere era but would include adjustments for earlier and later thin-atmosphere ice ages. For example, the late Ordovician-Silurian (450–420 mya) and late Carboniferous-Permian (300–252 mya) ice ages might warrant their own designations, while the current ice age beginning after the Cretaceous could mark the Cenozoic more precisely. Although such changes might be controversial, they reflect the gradual nature of climate shifts, which do not always align with the abrupt timing of mass extinctions.

Our current climate is not typical of the Phanerozoic eon. We are in an interglacial period within a series of ice ages that have persisted for millions of years. The present thin-atmosphere ice age climate is one of at least three glacial phases in the Phanerozoic eon, with others occurring near the end of the Ordovician period and during the late Carboniferous and Permian periods. Outside of these glacial intervals, the Phanerozoic was characterized by a stable, mild climate. Temperature differences between the equator and poles, sea level and mountain summits, or even between day and night were minimal. For much of the eon, the Earth experienced consistently balmy and moderate conditions.
Features of the Thick and Thin Atmospheres

Within the Phanerozoic eon, we can identify two cycles of thick and thin atmosphere eras, along with transitional periods between these extremes. The atmosphere shifted from being extremely thick to relatively thin during the Ordovician, Carboniferous, and Cretaceous/Paleogene periods, marked by significant carbon removal. Corresponding to these transitions, we find massive carbon deposits: limestone in the Ordovician, coal and limestone in the Carboniferous (for which it is named), and chalk in the Cretaceous, derived from calcium carbonate skeletons and shells of marine organisms.
Conversely, the transition from a thin to an extremely thick atmosphere occurred during times when shell-forming marine animals were nearly wiped out, such as after mass extinctions. With minimal biological removal of carbon dioxide, volcanic activity continued to pump CO₂ into the atmosphere, driving the shift toward a thicker atmosphere. These cycles illustrate the critical role of marine life in regulating Earth's atmospheric composition.
We can summarize our expectations for the two types of atmospheres and the transitional periods between the two as follows:
Characteristics of a Relatively Thin Atmosphere
1. Carbon Dioxide Levels: Nearly depleted, leading to a significantly thinner atmosphere.
2. Atmospheric Convection: With a thin atmosphere, three convection cells form in each hemisphere instead of one, creating distinct regional climates. Polar regions experience ice, and deserts form between 20 to 30 degrees latitude—contrasting sharply with the uniform climate of a thicker atmosphere.
3. Glacial Cycles: Over tens of thousands of years, polar ice remains continuous, with massive glacial movements extending across higher and middle latitudes.
4. Sea Level Fluctuations: Glaciers store vast amounts of water, causing sea levels to drop to their lowest during peak glacial extensions and rise as glaciers retreat.
5. Coal-Bearing Deposits: These sea level fluctuations produced numerous coal-bearing cyclothem deposits during the Carboniferous period.
6. Terrestrial Vertebrate Size: During the late Carboniferous and Permian periods, thin-atmosphere conditions limited terrestrial vertebrate size to no larger than modern examples.
7. Flying Vertebrates: Modern flying vertebrates are much smaller compared to those of the Mesozoic era when the atmosphere was significantly thicker.

Characteristics of a Thick Atmosphere
1. Carbon Dioxide Dominance: Carbon dioxide is the most abundant gas, resulting in a dense atmosphere.
2. Uniform Warm Climate: Temperatures are nearly the same everywhere, with no ice at the poles or mountain peaks.
3. Gigantic Terrestrial Vertebrates: The thick atmosphere supports terrestrial vertebrates as massive as whales.
4. Unique Dinosaur Anatomy: Terrestrial vertebrates, like dinosaurs, evolved strong, fish-like tails and disproportionately large rear legs to enhance speed and survival in the dense air.
5. Facilitated Flight: The dense air greatly aids flight, allowing even low-metabolism reptiles of the Mesozoic era to become the largest flying vertebrates in history.
Characteristics of a Thin Atmosphere Transitioning to a Thick Atmosphere
1. Triggered by Mass Extinction: The transition begins after a major extinction event that eliminates the marine animals responsible for removing carbon dioxide from the atmosphere.
2. Absence of Carbon Deposits: During the transition, no limestone or coal deposits form due to the lack of plant and animal species typically involved in their creation.
3. Gradual Recovery of Deposits: Dolomite formation resumes first, facilitated by simpler bacteria, followed millions of years later by limestone deposits as advanced, shell-creating marine animals slowly recover.
Characteristics of a Thick Atmosphere Transitioning to a Thin Atmosphere
1. Dominance of Carbon Removal: Underwater volcanic activity continues to release carbon dioxide, but marine life removes it at a faster rate, contributing to the widespread formation of carbonate deposits as atmospheric carbon dioxide levels decline.
2. Massive Carbonate Deposits: The transition is marked by extensive formations of carbonated rocks, such as limestone and dolomite, as marine life removes carbon dioxide from the atmosphere.
3. Thick-Like Atmosphere Climate Persists: Even as the atmosphere thins to a quarter or half its original thickness, the climate still resembles that of a thick atmosphere.
4. Decrease in Animal Size: Terrestrial animals gradually decrease in size as the transition approaches a thin atmosphere condition.
Timeline: Making Sense of the Geological Evidence
We can now look at a timeline for the major events of Phanerozoic eon based on our understanding of Earth having either a thick or a thin atmosphere:

1. Early to Middle Paleozoic Era:
- The Earth's atmosphere was most likely extremely thick throughout the entire early to middle Paleozoic era. While evidence of an ice age during the Silurian period might suggest a thinning atmosphere, a more plausible explanation for the glaciation is that Gondwana was positioned over the South Pole during this time.
- By the Devonian period and continuing into the Carboniferous, plants and arthropods, such as insects, had adapted to life on land, marking a significant step in the evolution of terrestrial ecosystems.
2. Carboniferous Period and Its Division:
- Carboniferous period further divided into Mississippian and Pennsylvanian periods.
- The Carboniferous period, named for its abundant coal deposits, coincides with a significant removal of carbon dioxide from the atmosphere as it transitioned from being thick to relatively thin.
- During this time, in the absence of competition from vertebrates, some insects grew exceptionally large (discussed in detail in the next chapter).

3. Glaciation during the Pennsylvanian Period:
- In West Virginia, there are 117 named coal seams from the Pennsylvanian period.
- The coal seams formed from the slow decay and compression of dead trees in low-oxygen swamp water, which arose from melting glaciers and the expanded coastal areas caused by low sea levels.
- During the Pennsylvanian period, glaciers advanced and retreated repeatedly, causing sea levels to rise and fall in sync.
- Scientists are uncertain why it was that during the Pennsylvanian period, and at other times when the atmosphere was thin, glaciers will advanced and retreated so frequently.
4. Thin Atmosphere During the Late Carboniferous and Permian Periods:
- Some amphibians grew large, but no terrestrial vertebrates exceeded the size of modern terrestrial animals.
- The thin atmosphere prevented vertebrates from evolving the ability to fly, while smaller insects overcame this barrier and diversified to exploit the aerial niche.
- Without flying vertebrate predators, many insects on the ground and in the air grew exceptionally large.
- Once vertebrates evolved flight in the thick atmosphere of the Mesozoic era, they outcompeted large insects, leading to the smaller insect sizes seen today. This is explored in detail in the next chapter.

5. Eruption of Siberian Traps:
- The volcanic emissions of the Siberian Traps wiped out 70% of terrestrial species and nearly all marine species.
6. Early Mesozoic Era:
- Following the P-T extinction, life on Earth was scarce for tens of millions of years.
- The Earth's atmosphere transitioned from relatively thin to extremely thick as volcanoes released carbon dioxide, which was no longer removed due to the near-total extinction of marine animals.
- There was no removal carbon dioxide from the atmosphere because there almost no terrestrial or marine life, and so there are no carbonate rocks or coal deposits during the first half of the Triassic period.
7. Signs of Recovery:
- In the oceans, bacterial life recovered and began converting carbon dioxide into dolomite, leading to the formation of the Dolomite of the Alps during the latter half of the Triassic period.
- Dinosaurs and many other forms of terrestrial life evolved during this time.
8. Triassic Ends with another Mass Extinction.
9. End of Jurassic Period
- While the graph indicates that the late Jurassic marked the peak of atmospheric thickness, an argument could be made that the atmosphere was thickest near the end of the Cretaceous period. Some of the largest sauropod dinosaurs lived during the late Jurassic, yet even larger sauropods appeared near the end of the Cretaceous. Similarly, the largest flying pterosaurs, such as Quetzalcoatlus, existed during the late Cretaceous.
10. Cretaceous Period
- During the Cretaceous Period, some dinosaurs weighed as much as a 100 tons, flying reptiles reached the size of small recreational airplanes, and giant flying birds terrorized the smaller dinosaurs.
11. K-T Mass Extinction
- In addition to wiping out the dinosaurs, the K-T extinction eliminated about 50–80% of marine species.
- Unlike the P-T extinction, which wiped out nearly all marine life for tens of millions of years and caused the atmosphere to grow thicker, the K-T extinction was followed by the rapid evolution of new marine animals within a few million years. These new species began removing carbon dioxide from the atmosphere at an even faster rate than before.
12. Thinning Atmosphere of the Cenozoic Era
- The mid-transition atmosphere remained thick enough to reduce terrestrial animal weight, enabling many of the new evolving mammals, like the giant Paraceratherium, to grow nearly as large as the dinosaurs they replaced.

13. Ice Ages of Cenozoic Era
- Near the end of the Tertiary period the atmosphere becomes thin enough that ice begins to form at the poles.
- Six million years ago, the atmosphere was still several times thicker than it is today. While it was not thick enough to significantly impact the size of terrestrial animals, it still allowed flying birds to grow exceptionally large. The gigantic flyer Argentavis, for example, achieved a wingspan two and a half times larger than the largest flying birds of today.
14. Present Day
- For the past 2.6 million years, we have been in an Ice Age due to the Earth’s atmosphere being relatively thin. Currently the Earth's climate oscillates roughly every 100,000 years between glacial periods and warmer interglacial times, such as the present.
External Links / References
Types of Rocks
- The three types of rocks - ZME SCIENCE
- Difference Between Rocks and Minerals - Geology In
- Difference Between Rocks and Minerals - Difference Between
- Different Types of Rocks- Difference Between
Fossils
- Fossil Evidence - UNDERSTANDING EVOLUTION
- What is a Fossil? - Roy Shephard and Robert Randell
- The Formation of Fossils - Science Views
- How Fossils Form - Fossil Museum
Fossils Hunting
- Finding a Dinosaur - Dick Wills: Dinosaur Hunter
- Fossil Hunting Sites
- Fossil Hunting Guide - Jurassic Park Terror
- Fossil Hunting Guide for United Kingdom
- Fossil Dig Sites Open To The Public in USA
- Fossil Hunting in the UK
Rockhounding
- How to Start Rockhounding
- State-By-State Rockhounding Location Guides
- How to Identify Minerals and Rocks
- 48 Great Tennessee Rockhounding Sites
Sedimentary Rock Formations and Relative Dating
- Relative dating - Science Learning Hub, New Zealand
- Sedimentary Rocks Formation and Fossils! - Christina Lornemark
- How do scientists date rocks and fossils? - Paleontological Research Institution
Dating Rocks
- How old are rocks? - Geological Survey, Ireland
- Absolute dating - Science Learning Hub, New Zealand
- Rock Formation II: Relative and Absolute Dating
- Radiometric Dating - UNDERSTANDING EVOLUTION
- Dating Rocks and Fossils - Peppe, D. J. & Deino, A. L.
Theory of Evolution Evidence
- Evidence for Evolution - KHAN ACADEMY
- Understanding Evolution - Lumen Learning
- Evidence for Evolution - Cliffs Notes
- Discoveries That Prove Evolution is Real - George Dvorsky
Punctuated Equilibrium
- Punctuated Equilibrium - PBS
- More on Punctuated Equilibrium - UNDERSTANDING EVOLUTION
- Fossil Evidence for Evolution - Open University
Population Growth Examples
- Population Growth and Regulation - Concepts of Biology
- 10 Real Life Examples Of Exponential Growth - Studious Guy
- Population Growth - Bio Ninja
Mass Extinctions
- Extinction - National Geographic Society
- THE EXTINCTION CRISIS - Center for Biological Diversity
- Timeline Of Mass Extinction Events - WORLD ATLAS
- Human Population Growth and Extinction - Center for Biological Diversity
Volcanoes Causing Mass Extinctions
- Cause of First Mass Extinction - TOHOKU UNIVERSITY
- Volcanoes and Mass Extinctions - Metageologist
- Mass Extinction of Dinosaurs - Princton University
- Triassic Extinction - MIT
Current Mass Extinction
- Sixth Mass Extinction - GUARDIAN
- Seven Billion Humans - EARTH IN TRANSITION
- What is Overpopulation? - Earth Eclipse
- Overpopulation Is Still the Problem - Alon Tal
Plants Need Carbon Dioxide: Symbiotic Relationships Between Plants and Animals
- Cellular Respiration and Photosynthesis - CK-12
- How do plants produce oxygen? - UCL
- Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants - Nature
- Making Food